The effect of Fe oxidation state on the specific surface area, Sm, of nontronite was studied using the <2-µm, Na+-saturated fraction of the SWa-1 and Garfield nontronite reference clays. The reduction of structural Fe3+ in the octahedral sheet of the nontronite decreased Sm as measured by the adsorption of 2-ethoxyethanol (ethylene glycol-monoethyl ether, EGME). The swellability in water of the nontronite also decreased during reduction. The amount of nonexchangeable Na+, on the other hand, increased with increasing Fe2+ content and was highly correlated with EGME adsorption (r = -.985). The relationship between Sm and Fe2+ was attributed to the collapse of partially or fully expanded layers to unexpanded layers.

This work investigates unit cell dimensions, crystal size and specific surface area of aluminous goethite that was progressively dehydroxylated to form hematite. Goethite synthesized from the ferrous system altered to hematite with DTGA maximum increasing from 236° to 273°C for 0 to 30.1 mole % Al-substitution. Unit cell dimensions of goethite and hematite decreased as Al-substitution increased and increased as excess OH increased. The crystallographically equivalent a axis of goethite and c axis of hematite were more sensitive than other axes to the presence of excess structural OH associated with Al-substitution. Specific surface area increased from 147 to 288 m2/g for goethite and from 171 to 230 m2/g for hematite as Al-substitution increased. An increase in specific surface area on heating goethite at temperatures between 200° and 240°C is related to a decrease in the size of coherently diffracting domains of goethite crystals and to the development of pore and structural defects associated with the formation of hematite. The decrease in specific surface area for heating temperatures above 240°C is attributed to the growth of hematite crystals by diffusion.

Methylene blue (MB) was adsorbed from aqueous solutions onto a kaolinite and four soil samples to determine the effects of MB dimerization on the measured surface area. Adsorption isotherms were prepared using four adsorbing solutions containing, respectively, 9, 46, 71, and 83% of MB molecules in the dimeric state. Langmuir-type isotherms were obtained in each case. The results indicate that equilibration occurs quickly. The aggregation state of MB molecules at the surface does not depend on the aggregation state in the initial adsorbing solutions, but on the final equilibrium concentration of MB. A comparison with the specific surface area measured by adsorption of ethylene glycol monoethyl ether indicates that MB adsorbs as a monomer, regardless of the aggregation number in solution. This result occurs owing to the strength of monomer-surface and monomer-monomer interactions. If monomer-surface interactions are favored, the MB dimer adsorbs in the monomeric form. If monomer-monomer interactions are favored, dimer adsorption may occur. The visible spectra of adsorbed molecules indicated that MB was present at the surface as a mixture of monomeric and dimeric species. These results suggest that dimers are formed in the contact region between two aggregating particles.

Estimations of the external specific surface areas (S.S.A.) by the N2-BET method of clay separates that were further fractionated by the high gradient magnetic separation technique revealed that the magnetic fractions had consistently lower S.S.A. compared to non-magnetic fractions. This phenomenon has been attributed, in the past, to the intimate association of Fe-oxides with silicate clays. It is the contention of this study that this reasoning is insufficient due to the following reasons. X-ray diffractograms (XRD) confirmed that heavy minerals were abundant in the magnetic fractions of these clays. Total chemical analyses and energy dispersive X-ray analyses showed that these heavy minerals contained Fe and Ti, which were not completely extracted by the dithionite-citrate bicarbonate (DCB) treatments. Crystallinity and quantity of these oxides in the different fractions did not show any relationships with the S.S.A. Lower S.S.A. were found in the magnetic fractions of both coarse and fine clays in the untreated as well as DCB-treated samples. The average particle density of the magnetic fractions was found to be higher than the non-magnetic fractions, resulting in an underestimation of the S.S.A. This underestimation was further proven when clay-sized illmenite (density = 4.79 Mg m−3) was found to have lower S.S.A. than quartz (density = 2.65 Mg m−3) and well-crystallized Georgia kaolinite (density = 2.61 Mg m−3), even though the illmenite particles were smaller in size compared to the kaolinite particles and similar in size compared to the quartz particles. It is, therefore, proposed that the specific surface areas should be expressed either on a volumetric basis or corrected for differences in density to avoid underestimations when heavy minerals are present in the samples.

Hydroxyaluminosilicate (HAS) ions prepared from hydroxy-Al (HyA) ions and orthosilicic acid at different NaOH/Al molar and Si/Al atomic ratios were fixed in the interlayer spaces of vermiculite (Vt). The electric charge and surface characteristics of HAS-Vt and HyA-Vt complexes formed were investigated in the pH range of 4 to 8. At pH 4 to 6, the magnitude of negative charge (CEC) of HAS-Vt and HyA-Vt complexes was drastically reduced by a HAS- or HyA-interlayer formation of Vt. At pH 7 to 8, especially in NaOH/Al molar ratio of 2.5, the magnitude of negative charge was from 62 to 89% of CEC in untreated Vt, suggesting that part of HAS or HyA ions fixed on Vt was excluded from its interlayer spaces. The positive charge did not develop on HAS-Vt and HyA-Vt complexes at pH between 4 and 8. The fixation of HAS or HyA ions on Vt caused the significant reduction of its total and internal surface areas as well as the slight increase of its external surface area. The HAS- or HyA-fixation in the interlayer spaces of Vt was confirmed by X-ray diffraction analysis. Our results provided evidence of a possibility that Vt could fix HAS ions in the same way as HyA ions, transforming to chloritized-Vt. These interlayer materials could play a significant role in modifying the surface and mineralogical properties and cation exchange capacity of clays and soils.

The increasing exploration and exploitation of hydrocarbon resources hosted by oil and gas shales demands the correct measurement of certain properties of sedimentary rocks rich in organic matter (OM). Two essential properties of OM-rich shales, the total specific surface area (TSSA) and cation exchange capacity (CEC), are primarily controlled by the rock’s clay mineral content (i.e. the type and quantity). This paper presents the limitations of two commonly used methods of measuring bulk-rock TSSA and CEC, ethylene glycol monoethyl ether (EGME) retention and visible light spectrometry of Co(III)-hexamine, in OM-rich rocks. The limitations were investigated using a suite of OM-rich shales and mudstones that vary in origin, age, clay mineral content, and thermal maturity.

Ethylene glycol monoethyl ether reacted strongly with and was retained by natural OM, producing excess TSSA if calculated using commonly applied adsorption coefficients. Although the intensity of the reaction seems to depend on thermal maturity, OM in all the samples analyzed reacted with EGME to an extent that made TSSA values unreliable; therefore, EGME is not recommended for TSSA measurements on samples containing >3% OM.

Some evidence indicated that drying at ⩾200°C may influence bulk-rock CEC values by altering OM in early mature rocks. In light of this evidence, drying at 110°C is recommended as a more suitable pretreatment for CEC measurements in OM-rich shales. When using visible light spectrometry for CEC determination, leachable sample components contributed to the absorbance of the measured wavelength (470 nm), decreasing the calculated bulk rock CEC value. A test of sample-derived excess absorbance with zero-absorbance solutions (i.e. NaCl) and the introduction of corrections to the CEC calculation are recommended.

The total specific surface area (TSSA) and smectitic layer charge (Qs) calculated from the structural formulae and unit-cell dimensions of 12 pure smectite samples were used as a reference in the design and evaluation of TSSA and Qs measurement techniques based on cation exchange capacity (CEC), H2O retention at 47% RH, and ethylene glycol monoethyl ether (EGME) retention. A thermogravimetric analysis-mass spectrometry (TGA-MS) technique was used to study the release of H2O from smectite on heating, and to introduce a correction for H2O remaining in the smectite after heating to 110°C, because the sample weight at this temperature has been used routinely as a reference in CEC and EGME sorption measurements. A temperature of 200°C was found to be the optimum reference for such measurements.

A good agreement between Qs from the structural formula and from CEC was obtained when this correction was applied. The TSSA of smectite was measured with similar accuracy (mean error of ±5–7%) by three techniques: (1) using mean H2O coverage; (2) using mean EGME coverage; and (3) using a combination of H2O coverage and CEC. A reduction of the mean error from 5–7% to 4% can be obtained by averaging these measurements, and a further reduction to 3% by introducing corrections for the dependence of H2O and EGME coverage on layer charge. The study demonstrates that Ca2+-smectite samples at 47% RH have H2O contents corresponding to 88–107% of the theoretical mass of a monolayer and offers an explanation of this variation.

Maghemite (γ-Fe2O3) is a ferrimagnetic Fe oxide commonly found in tropical and subtropical soils, especially in the topsoil where it is usually a product of burning. Isomorphic substitution (IS) of the Fe in maghemite by different metals (mainly Al3+) can modify its mineralogical and chemical attributes, and these modifications may be important to understanding the formation and properties of this mineral in soils and sediments. The objective of this work was to evaluate the crystallochemical alterations of synthetic, Al-substituted maghemites prepared by the precipitation of magnetites from alkaline aqueous media containing FeSO4·7H2O with increasing amounts of Al2(SO4)3·7H2O to obtain hypothetical Al3+ for Fe3+ substitutions ranging from 0.0 to 40.0 mol %. The Al-substituted magnetites were washed and dried, and then heated to 250ºC for 4 h to form yellowish red maghemites that were characterized by total chemical analysis, X-ray diffraction, specific surface area (SSA), mass-specific magnetic susceptibility, infrared spectroscopy, transmission electronic microscopy, and color. Increasing Al3+ substitution to an experimental maximum of 15.9 mol % decreased both the a0 dimension of the cubic unit cell (a0 = 0.8339 − 396.157 × 10−16 Al, r2 = 0.99) and the mean crystallite dimension (MCD = 76.4–3.15Al, r2 = 0.79) of the maghemites. With the decrease in MCD came a more yellowish color, an increase in SSA, and a decrease in crystallinity as measured through extraction of the samples with acid ammonium oxalate. The mass-specific magnetic susceptibility of the maghemites increased with Al3+ substitution up to 5.3 mol % and then decreased with further replacement of Fe by Al. Solid-phase aluminum in excess of 16 mol % substitution appeared to occur as a separate, poorly crystalline phase that was X-ray amorphous.

The nature of component layers of mixed-layer illite-smectite and their possible evolution in the course of illitization have been debated since the 1960s. The present study is a new attempt to solve these problems, using samples collected from diverse geological formations around the world. Twenty three purified illite-smectites from bentonites and hydrothermal rocks, covering the complete range of expandability, were analyzed chemically, including ${\text{NH}}_4^{+}$\$\end{document} determination, and their structural formulae were calculated. The exchangeable cations (EXCH) were plotted vs. the fixed cations (FIX) yielding the following experimental regression:$\text{EXCH}=-0.43\times \text{FIX}+0.41\left({\text{R}}^2=0.98\right)$\$\end{document}

FIX and EXCH depend on the charge of the illite (Qi) and smectite (Qs) interlayers, and the fractions of these interlayers in the bulk clay leading to: $\text{EXCH}=-Q_{\text{s}}\text{/}{Q}_{\text{i}}\times \text{FIX}+Q_{\text{s}}$\$\end{document}

Analysis of these relations and independent measurements of the total specific surface area (TSSA) indicate that the layer charges of both types do not change in the course of illitization. The smectite layer charge is equal to 0.41 and the illite layer charge is equal to 0.95 per O10(OH)2. End-member illite has a well defined composition that is close to intermediate between muscovite and phengite, with fixed cations content greater than that specified in the AIPEA classification of layer silicates:${\text{FIX}}_{0.95}\left({\text{Si}}_{3.25}{\text{Al}}_{0.75}\right)\left({\text{Al}}_{1.81}{\text{Fe}}_{0.01}{\text{Mg}}_{0.19}\right){\text{O}}_{10}{\left(\text{OH}\right)}_2$\$\end{document}

The established relationship allows the calculation of the mean number (N) of 2:1 layers in all fundamental particles and also the fraction of smectitic layers (fs) from FIX:$N=Q_{\text{i}}\text{/}(Q_{\text{i}}-\text{FIX})f_{\text{s}}=(Q_{\text{i}}-\text{FIX})\text{/}{Q}_{\text{i}}$\$\end{document}

N and fs can be used to calculate TSSA, and all three parameters can also be calculated from cation exchange capacity and from X-ray diffraction peak positions, utilizing the regressions established here.

The Clay Minerals Society published a complete characterization scheme for its ‘Source Clays’ but not for its ‘Special Clays’. To address this issue, the specific surface areas (SSAs) of the 16 special clays from The Clay Minerals Society were determined using the Brunauer, Emmett and Teller (BET) method of adsorption of an inert gas. Two BET measurements were performed for each of the 16 special clays, and the average BET SSA of each of the special clays was determined. The BET SSA of cookeite is reported for the first time. In the present study, special clays from The Clay Minerals Society are classified under three groups based on their BET special surface area values as Group-I special clays, with BET values of 0.1–10 m2/g, Group-II special clays, with BET values of 10–100 m2/g, and Group-III special clays, with BET values >100 m2/g. Comparisons which proved interesting were the those involving the mixed-layer clays and the synthetic clays. The systematic approach employed in this paper will allow for better comparisons to be made between different clays and will provide a comprehensive database for future applications of such material (e.g. as catalyst carriers, as adsorbents in waste treatments, etc.).

An experimental program was conducted to investigate the water-vapor sorption characteristics of smectite and kaolinite mixtures. End-member smectite and kaolinite were slurry-mixed together at mass-controlled ratios corresponding to 0%, 20%, 50%, 70%, 80%, 90% and 100% smectite. Vapor desorption isotherms for the mixtures were measured at 24°C for relative humidity (RH) ranging from ∼95% to 0%.

Results show that the amount of water adsorbed by the clay mixtures at a given RH increases systematically with increasing smectite content. Derivative analysis of the sorption isotherms shows evidence of transitions between the two-, one- and zero-layer hydrate-states for the smectite-rich mixtures. The transitions become less apparent as the smectite content decreases. Monolayer coverage, specific surface area, and heat of adsorption were estimated from the isotherms using BET theory. It is shown that monolayer coverage and specific surface for the clay mixtures can be reasonably approximated by weighted averaging of the end-member clay properties. General methodologies are presented for predicting the sorption behavior (i.e. soil-water characteristics) and effective specific surface area from measurements of the end-member sorption isotherms.

The extremely toxic protein, ricin, is derived from castor beans and is a potential terrorist weapon. Adsorption to clays might minimize the environmental persistence and toxic effects of this toxin. Ricin adsorption to clay minerals was measured using batch adsorption isotherms. Enzyme-linked immunoassay methods were used to quantify aqueous ricin concentrations. Montmorillonite, sepiolite and palygorskite effectively adsorbed ricin from aqueous solutions and yielded mostly Langmuir-type isotherms. The monolayer adsorption capacity from a Langmuir equation fit at pH 7 was 444 g ricin/kg for montmorillonite (SWy-2), but was only 5.6 g ricin/kg for kaolinite (KGa-1b). Monolayer capacities for sepiolite (SepSp-1) and palygorskite (PFl-1) at pH 7 were 59.2 and 58.1 g ricin/kg. The high-charge montmorillonite (SAz-1) effectively adsorbed ricin at pH 7, but yielded a linear isotherm with K = 5530 L/kg. At pH 5, both montmorillonites (SWy-2 and SAz-1) yielded Langmuir-type isotherms with monolayer capacities of 694 and 641 g ricin/kg. Clay samples with higher cation exchange capacities generally adsorbed more ricin, but adsorption also followed specific surface area. X-ray diffraction of <2 μm SWy-2 treated with 470 g ricin/kg indicated expansion up to 34.6 Å at buffered pHs of 4 and 7, but not at pH 10. Furthermore, ricin adsorption was greatest at pH 4 and 7, but minimal at pH 10. Treatment with 1.41 kg of purified ricin/kg clay at pH 5 yielded a 35.3 Å peak and adsorption of ~1.2 kg ricin/kg. Similar treatment with lower-purity ricin yielded less expansion and lower adsorption. The 35.3 Å peak interpreted either as a d002 or d001 reflection indicates a 70.6 Å or a 35.3 Å ricin/SWy-2 complex. This implies that adsorption and air drying have compressed interlayer ricin molecules by 18 to 65%. Effective ricin adsorption by montmorillonite suggests that it could be used to minimize the toxic effects of dispersed ricin.

Specific surface area measurements of The Clay Minerals Society source clays were made by the Brunauer, Emmett and Teller (BET) method of adsorption of nitrogen gas. Two replicate measurements of specific surface area were performed for each source clay. All pair values were within 3%, which is very good agreement for this type of measurement.

Microwave irradiation as a means for heating bentonites during acid activation has been investigated in the past but it has never been optimized for industrial applications. The purpose of this study was to apply a factorial 23 experimental design to a Serbian bentonite in order to determine the influence of microwave heating on the acid-activation process. The effect of acid activation under microwave irradiation on the textural and structural properties of bentonite was studied as a model reaction. A mathematical, second-order response surface model (RSM) was developed with a central composite design that incorporated the relationships among various process parameters (time, acid concentration, and microwave heating power) and the selected process response of specific surface area of the bentonite. The ranges of values for the process parameters chosen were: time, 5–21 min; acid concentration, 2–7 M; and microwave heating power, 63–172 W. The effect of individual variables and their interaction effects on the textural and structural properties of the bentonite were determined. Statistical analysis showed that the duration of microwave irradiation was less significant than the other two factors. The model showed that increasing the time and acid concentration improved the textural properties of bentonites, resulting in increased specific surface area. This model is useful for setting an optimum value of the activation parameters for achieving the maximum specific surface area. An optimum specific surface area of 142 m2g−1 was achieved with an acid concentration of 5.2 M, activation time of 7.4 min, and microwave power of 117 W.

Natural tuff samples in western Anatolia (Türkiye) originating from Miocene rhyolitic–pyroclastic rocks with >80 wt.% heulandite/clinoptilolite zeolites were investigated for their surface characteristics determined according to nitrogen adsorption after degassing at 150°C (specific surface area, pore volume and pore diameter). Additionally, these surface characteristics were correlated with the cationic compositions of the heulandite/clinoptilolite group minerals. The examined samples were characterized by two main pore diameters that were not related to specific surface area and pore volume but were partially related to the types and occupancy of extra-framework cations. One set of samples has a pore diameter of ~24 Å, total cation content (Na + K + Ca + Mg) ranging from 3.46 to 4.40 and a (Na + K)/(Ca + Mg) ratio ranging from 0.34 to 0.92. The total cation contents and (Na + K)/(Ca + Mg) ratios of the remaining samples with a pore diameter of ~37 Å are 4.30–5.08 and 1.48–2.85, respectively. After degassing at 300°C, there is a slight difference in the pore diameters of these two sets of samples (~37 and 38 Å). The pore sizes of the samples with a (Na + K)/(Ca + Mg) ratio < 1 (heulandite composition) increased from 24 to 36–38 Å with increasing degassing temperature, whereas the pore sizes of the samples with a (Na + K)/(Ca + Mg) ratio > 1 (clinoptilolite composition) increased from 37 to only 38–39 Å. However, there is no correlation between the Si/Al ratios and the cation-exchange capacities of the samples and their surface characteristics obtained by degassing at the two temperatures.

Allophane-related aluminium silicates with various chemical compositions were synthesized using a hydrothermal reaction with inorganic reagents as starting solutions. The X-ray diffraction traces of the synthesized allophanes showed broad reflections centred at 0.34 and 0.23 nm, which were attributed to the imogolite-like structure of the allophanes. Energy-dispersive X-ray measurements confirmed that the Al-rich materials were synthesized as targeted. The specific surface area of the synthesized allophanes was 313–500 m2 g–1, which is greater than that of a natural allophane (248 m2 g–1). The amount of nitrate adsorbed on the synthesized allophanes tended to increase as the Al content increased. The maximum amount of nitrate adsorbed was 2.07 mmol g–1 at pH ~2, which was comparable to that of a common anion-exchange material. A possible adsorption mechanism for nitrate at lower pH levels is the weak NO3– interaction of the positively charged surface of Al-OH2+ sites.

Clay-mineral type and content, bulk mineralogical composition and alteration of bentonite are very important factors for the ultra-long-term stabilization of barriers and backfills in radioactive waste disposal. This study investigates the effects of clay-mineral type and content on the swelling characteristics and permeability of bentonite–sand mixtures with clay minerals using one-dimensional swelling-pressure and constant-pressure permeability tests. The hydraulic conductivity of bentonite–sand–clay mineral mixtures increased with increasing content of non-swelling alteration products of montmorillonite. Furthermore, hydraulic conductivity was comparable to that determined with the Kozeny–Carman equation for a specific surface area, suggesting that hydraulic conductivity may be estimated based on the abundance of expected alteration products of montmorillonite. This study provides a basis for evaluation of the hydraulic conductivity of bentonite–sand mixtures with known quantities of expected alteration products of montmorillonite.

The transformation of schwertmannite to goethite was studied by ageing one synthetic and five natural schwertmannites in water at room temperature. Additionally, one synthetic and two natural schwertmannites were kept at variable pH (2, 4, 6 and 8). After one year, only the synthetic sample and one natural schwertmannite had transformed to goethite. However, the oxalate solubility of Fe and trace elements in all the samples decreased, whereas the total Fe/S ratios and specific surface areas of all samples increased. Arsenic and organic matter appeared to suppress the schwertmannite-to-goethite phase transformation. At pH 2, synthetic schwertmannite fully-transformed to goethite, but at pH 4–6 only minor transformation occurred. Depending on pH, many trace elements were released into solution during ageing of the natural schwertmannites. In general, Co, Mn, Zn and Si were released to solution, whereas As was enriched in the remaining iron oxide fraction. Al was dissolved at pH <4.

There is interest in exploiting and developing natural resources, particularly deposits of natural clays. Senegal has several clay mineral deposits for which chemical and mineralogical compositions have been little studied. Some of these natural materials are nowadays used in pottery and ceramics. To extend applications, a better basic knowledge is required and, for this objective, the raw clay and separated <2 μm clay fraction from Keur Saër (Senegal) were subjected to chemical and mineralogical studies. Several techniques including X-ray diffraction (XRD), thermal analysis (TG-DTA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), N2 adsorption-desorption isotherms, cation exchange capacity (CEC) measurements and solid state nuclear magnetic resonance (NMR) have been used to characterize the material. It was found that the raw clay and the separated clay fraction consist of a mineral mixture in which kaolinite is the main component. 29Si and 27Al MAS-NMR spectra show the presence of silicon atoms linked to three other silicon atoms via an oxygen atom and six coordinated Al atoms. Significant increases in the specific surface area and cation exchange capacity were observed on purification, reaching a maximum of about 73.2 m2g–1 and 9.5 meq/100 g for the separated fine clay fraction while the values for the raw material were around 28.9 m2g–1 and 7.3 meq/100 g.

Thermal analysis (DTA-TG-DTG), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, nitrogen adsorption and mercury intrusion porosimetry techniques were used to assess the structural changes induced upon heating of two hydrotalcite-phenol (trichloro- and trinitrophenol, HT-TCP and HT-TNP) complexes, and the results were compared with those obtained for the original hydrotalcite (HT) sample. The DTA revealed thermal effects that depended on the nature of the interlayer ion in the complexes. The total weight loss (TG-DTA) increased from 37% for the original HT to 40% for HT-TCP and 77% HT-TNP, as the amount of phenol increased. The XRD and FTIR spectroscopy showed that the calcination product (550°C) of the HT-phenol complexes was indistinguishable from that formed from the original HT. Since HT-phenol complexes were prepared by phenol adsorption on calcined HT, our results confirm the recyclability of HT-like compounds as sorbents for phenols.